PURPOSE: To obtain the title compound useful as an intermediate for synthesizing drugs such as vitamin E by the use of small amounts of reagents sold on the market at high reaction rate in high yield by oxidizing 2,3,6-trimethylphenol with oxygen in the presence of copper chloride and a manganese compound as a catalyst in a specific solvent.

CONSTITUTION: 2,3,6-Trimethylphenol is oxidized with oxygen or an oxygen- containing gas in a solvent containing a polyethylene glycol alkyl ether shown by the formula (n is 1-3; R is CH3, C2H5, C3H7 or C4H9; R' is H, CH3, C2H5, C3H7 or C4H9) at 20-120°C, preferably 60-100°C to give the aimed compound. Ethylene glycol dimethyl ether, etc., are used as the solvent. An organic solvent such as dimethylformamide may be subsidiarily used and the amount of the polyethylene glycol alkyl ether used is preferably ≥80vol% based on the total solvents.